SANTA CLARA -- Rookie safety Eric Reid got a chance this week to prepare for the time when he is in total charge of the 49ers' secondary.

Reid took on more responsibility on the days when veteran safety Donte Whitner was absent from organized team activities.

"They expect me to make more calls, but it goes back to studying my plays every day," Reid said. "That's realistic because in this game somebody can get hurt and you never know when it's going to happen. If he's not in, it's my turn to step up and make some calls."

The pressure is on Reid to win the job to replace All-Pro Dashon Goldson, who signed with the Tampa Bay Buccaneers in March as a free agent. The 49ers traded up 13 spots to select the former LSU star with the No. 18 overall selection with the expectation that he would distinguish himself as a starter.

During a practice session this week that was open to the media, Reid split time with the first unit with Craig Dahl and C.J. Spillman in Whitner's absence.

Reid appears to be adapting as well as can be expected for a rookie who must learn both safety positions, as well as the responsibilities of everyone in front of him.

"Technically, I'm at free (safety), but depending on motion or the change in strength of the offense, your responsibility could change to strong," Reid said. "The safeties have to know both positions."

Reid said he feels as if he has adjusted well to the speed of the game. Now, he has to adjust to the speed of the thinking that goes along with a thicker playbook and the necessity for quicker decisions.

"Every day we're installing new plays, so sometimes it's hard to remember something new and fresh and have to do it on the field," Reid said. "There have been a couple times I was wrong on that one, but coach (Ed Donatell) comes up to me and tells me what I need to do to fix it, and I got to do it right the next time.

"It goes back to the speed and mental aspect. If you're half-stepping, Vernon Davis is running by you from the tight end (position) or the X receiver is running by you. If you don't know what you're doing, you're going to mess up the guy next to you. So you got to be on your toes at all times."